Process for the vaporization of alkyl aromatic liquid feeds with removal of high boilers

ABSTRACT

An improved process for the vaporization of liquid hydrocarbon feeds wherein high boiling components present in the liquid feed are removed from the feed prior to subsequent processing comprising heating the liquid feed to a temperature at, but just below, its dew point, introducing the thus heated feed into a separation zone where any high boiling liquid components are removed and removing a substantially dry gaseous feed from the separation zone. The process minimizes fouling in heat exchangers and other indirect heating systems caused by coking of entrained high boiling liquids in the vaporized feed.

United States Patent Smalling, Jr.

[ 51 July25, 1972 [54] PROCESS FOR THE VAPORIZATION OF ALKYL AROMATIC LIQUID FEEDS WITH REMOVAL OF HIGH BOILERS [72] Inventor: Claude W. Smalling, Jr., 101 Larkspur,

- Alvin, Tex. 7751 1 [22] Filed: May 15, 1970 211 Appl. No.: 37,792

[52] US. Cl ..260/672 NC, 208/113, 203/40, 203/27, 202/197, 203/22 [51] Int. Cl ..C07c 3/58 [58] Field of Search ..260/672 NC; 208/353,365; 203/40, 22,27, 88; 196/134; 202/197, 177, 182

[56] References Cited UNlTED STATES PATENTS 3,288,876 11/1966 Hammond ..260/672 NC 3,193,592 7/1965 Eubank..... ..260/672 NC 3,188,359 6/1965 Lempert.... ..203/22 1,546,345 7/1925 Laird ...203/22 CONDENSER FROM REBOILER 3,223,746 12/1965 Hammond ..260/672 NC 3,159,567 12/1964 Young ....260/ 672 NC 2,953,514 9/1960 Wilkins ....260/672 NC 3,374,280 3/1968 Carr et al. ..260/672 NC 3,310,593 3/1967 Nelson ..260/672 NC Primary ExaminerWilbur L. Bascomb, Jr. Attorney-Thomas 13. Leslie, L. Dan Tucker and Neal E. Willis [57] ABSTRACT An improved process for the vaporization of liquid hydrocarbon feeds wherein high boiling components present in the liquid feed are removed from the feed prior to subsequent processing comprising heating the liquid feed to a temperature at, but just below, its dew point, introducing the thus heated feed into a separation zone where any high boiling liquid components are removed and removing a substantially dry gaseous feed from the separation zone. The process minimizes fouling in heat exchangers and other indirect heating systems caused by coking of entrained high boiling liquids in the vaporized feed.

7 Claims, 2 Drawing Figures TO REED/LEI? T0 FIRED HE, AND REAC' FROM REACT PATENTEDJUL25 I972 fiLTE k 2 Q u 1 a w it A\ Lu Q.

3 v Eff v v RUN Q, X RUN 2 2 5 v A RUN 3 i I n RUN 4 a m O RUN 5 E v v V RUN 6 ti o RUN 7 i a A v DAYS SINCE CLEAN/N6 HEAT EXCHANGER FROM REBOILER CONDENSER CLAUDE 14 T0 REBO/LER T0 FIRED HEATER AND REACTOR FROM REACTOR INVENTOR. SMALL/NQJR.

af p ii.

ATTORNEY PROCESS FOR THE VAPORIZATION OF ALKYL AROMATIC LIQUID FEEDS WITH REMOVAL OF HIGH BOILERS BACKGROUND OF THE INVENTION The present invention relates to the vaporization of liquid hydrocarbon feeds. More particularly, the present invention relates to the removal of high boiling liquids from vaporous feeds so as to reduce coking in heat exchangers and other indirect heating systems.

In numerous different processes employed in both the chemical and refining industries, it is necessary that a liquid hydrocarbon feedstock be vaporized and preheated prior to use in the process. For example, in the cracking of petroleum feedstocks, a condensate is vaporized and then passed through fired cracking furnaces where the condensate is thermally cracked to form lighter components. Likewise, in hydrodealkylation processes wherein an aromatic feedstock containing components such as alkyl benzenes and/or alkyl naphthalenes and hydrogen are fed into a hydrodealkylation zone to convert the alkyl benzenes to benzene and the alkyl naphthalenes to naphthalene, it is necessary that the feed to the hydrodealkylation zone be vaporized and heated prior to introduction into the zone. In the above two mentioned processes and many others involving hydrocarbon feedstocks of numerous components many of which are boiling at relatively high temperatures, i.e. above 550 F., it is not uncommon to find that the feedstock contains minor amounts of components boiling at extremely high temperatures as for example above 600 F. Since it is not technically feasible to heat the feed sufficiently to vaporize these high boiling components, they generally remain as entrained droplets in the vaporized feed. Optimization of heat balance in a system requires that numerous heat exchange systems by employed whereby a hotter medium can be used to heat a cooler medium which itself must be heated. Typical of such heat exchange systems are the common tubeshell type exchangers wherein one medium is passed through a plurality of tubes and another medium, generally at lower temperatures, is passed in the space around and between the tubes such that the medium flowing through the tubes gives off its heat to the medium flowing in the space between and around the tube. When such heat exchangers and other indirect heating equipment are used to heat vapors which have entrained high boiling hydrocarbons, it frequently happens that the high boiling hydrocarbon liquids upon contacting the heated tubes in the exchanger will coke. This of course drastically reduces the heat exchange between the vaporized feed and the heating medium requiring additional utility heat to bring the feed to the desired temperature. Depending on the amount of the high boiling hydrocarbon liquid present in the vapor and the temperature of the tubes, the coking rate may be excessive with the result that the exchanger soon fouls terminating the process until the exchanger can be cleaned. As will be readily appreciated, this down time" represents significant economic loss both in terms of maintenance and loss of production.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an improved process for the vaporization of liquid feeds.

It is another object of the present invention to provide a process for the vaporization of liquid feeds containing a major proportion of hydrocarbons.

Yet another object of the present invention is to provide a process for the vaporization of liquid feeds containing a major proportion of hydrocarbons wherein high boiling liquid hydrocarbons are removed from the vaporized feed.

An important object of the present invention is to provide a process for the removal of high boiling hydrocarbons from vaporized feeds which high boiling hydrocarbons would cause coking of heat exchange systems if allowed to remain in the vaporized feed.

Another object of the present invention is to provide a process for the removal of high boiling hydrocarbons from a vaporized alkyl aromatic feed used in a hydrodealkylation reaction.

These and other objects of the present invention will become apparent from the drawings, the description given herein and the appended claims.

In accordance with the improvement described herein, a liquid feed containing a major proportion of hydrocarbons is heated to a temperature just below its dew point. The thus heated liquid is then introduced into a separation zone or entrainment separator wherein any high boiling hydrocarbon liquids are removed from the heated feed. A gaseous feed, substantially free of entrained high boiling hydrocarbon liquids, is then removed from the separation zone and further processed as desired.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a diagrammatic flow diagram illustrating the im proved process of the present invention.

FIG. 2 is a graph showing the improved results in using the process of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference is now made to FIG. 1 for a detailed description of the operation of the process of the present invention as used in a hydrodealkylation scheme, it being understood that the improvement is not so limited. A liquid feed containing a major proportion of hydrocarbons is introduced via line 10 into the shell side of heat exchanger 11 where it is partially heated by a warmer medium (whose source will be described later) passing through the tube side of heat exchanger 11. The effluent from the shell side ofexchanger 11 is removed via line 12 and enters the shell side of exchanger 13 where it is heated to a point just below its dew point. The feed entering the shell side of heat exchanger 13 is heated by a medium passing through the tube side of heat exchanger 13 which is the same medium which passes through the tube side of heat exchanger II. The feed leaving the shell side of exchanger 13 passes via line 14 into separator 15 wherein any high boiling point liquids remaining in the feed at the temperature to which it has been heated are removed from the vaporized feed. The high boiling point liquids thus removed are rejected from the bottom of separator 15 through line 16. A substantially dry gaseous feed is removed overhead from separator 15 via line 17 and passed to the shell side of exchanger 18 where it is heated by the previously referred to medium which.is passing through the tube side of exchanger 18. The heated vapor from the shell side of exchanger 18 is removed via line 19 and enters the shell side of exchanger 20 to be further heated by the medium referred to above which is passing through the tube side of exchanger 20. The further-heated vaporized feed from the shell side of exchanger 20 is removed via line 21 and is sent to a tired heater and then a hydrodealkylation reactor (both of which are not. shown). The effluent from the hydrodealkylation reaction is at a very high temperature and provides an excellent source of heat to raise the temperature of the incoming feed. Accordingly, the medium which is passed through the tube side of exchangers 20, l8, l3 and 11 and which serves as the heat source for the feed to the hydrodealkylation reaction comprises the effluent from the hydrodealkylation reactor. This efi'luent enters the tube side of exchanger 20 via line 22, passes via line 23 through the tube side of a distillation column reboiler (not shown), passes via line 27 from the reboiler through the tube side of exchanger 18 into line 24, enters the tube side of exchanger 13 exiting via line 25 and enters the tube side of exchanger 11 exiting via line 26 through which it passes to a condenser or the like. As can be seen, by this scheme, the rather large heat content of the reactor effluent is effectively utilized to heat the incoming feed to the reactor.

ln heretofore proposed processes, separator 15 was not present in the system with the result that unvaporized highboiling liquid hydrocarbons were allowed to pass through the system and to come in contact with the extremely hot tubes in heat exchangers l8 and 20, particularly exchanger 20. Upon contacting these extremely hot tubes in exchanger 20, the unvaporized, high-boiling liquid hydrocarbons were caused to undergo coking with the result that over a relatively short period of time, the shell side of exchanger became quite fouled such that the feed leaving the shell side of exchanger 20 was at an undesirably low temperature due to inefficient heat exchange. This of course required additional preheating prior to introduction into the hydrodealkylation reactor. Furthermore, the fouling of these exchangers was so rapid as to limit their usefulness to a relatively short period of time before they would have to be disassembled and cleaned.

The effectiveness of the process described herein is best shown by the following non-limiting example. The system used in the example was substantially the one shown in the accompanying drawing.

EXAMPLE The alkyl aromatic feed used in this example contained 54.8 percent by weight toluene, 45.2 percent by weight unifined alkyl naphthalene concentrate and had a dew point of 470 F. The material fed to the inlet of the shell side of exchanger 11 comprised of the alkyl aromatic feed, recycle, hydrogen and methane had the following average composition:

Component Percent Toluene 27.3 Unifined Alkyl Naphthalene Concentrate 22.5

' Recycle l l.() Hydrogen l 1.4 Methane 27.3

*Recycle of unconverted feed from hydrodealkylation reactor effluent.

flow from the shell side of exchanger 13 passing directly into the shell side of exchanger 18. Run 7 was conducted with the separator described above present in the system.

As can clearly be seen from HO. 2, the process of the present invention is extremely effective in reducing the fouling in heat exchanger 20. Note that the temperature of the feed exiting from the shell side of exchanger 20 remains at a much higher level for a longer period of time. As pointed out above, this is quite important inasmuch as it reduces the load on the fired preheater used to heat the feed from the shell side of exchanger 20 prior to its introduction into the hydrodealkyla tion reaction zone. Of greater importance is the fact that the process herein eliminates the frequent shutdowns normally required in order to clean the heat exchange system.

While the invention has been described with reference to an alkyl aromatic feed which is being heated prior to its introduction into a hydrodealkylation zone, it is to be understood that the invention has application to any system where a liquid feed containing a preponderance of hydrocarbons is to be vaporized prior to further processing. An example of such a usage is the vaporization of a petroleum condensate prior to its being introduced into a cracking zone to convert it into lighter hydrocarbon such as ethylene, propylene, etc. The invention finds particular application, however, for use as part of a hydrodealkylation process wherein an alkyl aromatic feed containing such materials as toluene, alkyl naphthalenes, etc. is fed concurrently with hydrogen into a high temperature hydrodealkylation zone. A typical alkyl aromatic feed will comprise from about 50 percent to about 70 percent by weight toluene and from about 30 percent to about 50 percent by weight unifined alkyl naphthalene concentrate and have a dew point ranging from about 450 to about 480 F. It is to be understood however that alkyl aromatic components can be used. For example a feed consisting essentially of only unifined alkyl naphthalene concentrate can be used. if desired.

The term alkyl aromatic feed as used herein refers to any liquid feedstock containing compounds having aromatic rings with alkyl side chains attached thereto. Examples of such compounds include toluene, the xylenes, trimethyl benzenes, ethyl and diethyl benzenes, and the numerous alkyl naphthalenes such as methyl naphthalene, dimethyl naphthalene, ethyl naphthalene, diethyl naphthalene, etc. The term unifined alkyl naphthalene concentrate refers to a feedstock which initially contained thermally reactive compounds similar, but not limited to styrenes, indenes, diolefins, etc. which have been selectively hydrogenated to saturate the reactive carbon to carbon double bonds present in such thermally reactive compounds.

As explained above, the process of the present invention contemplates heating the feed material to a temperature just below its dew point, i.e. at a temperature at which the feed is substantially vaporized. Although the exact temperature to which the feed is heated will depend upon the composition of the feed, it has been found that whatever the composition of the feed, it should be heated to a temperature which is less than 10 F. below the known dew point of the feed. Failure to heat the feed to this point results in an excessive amount of liquid being present and consequently excessive loss of liq uid in the separator.

The separation zone can comprise any system wherein the entrained high boiling liquid is removed from the substantially vaporized feed. Suitable separation systems include gravity settlers, cyclone or centrifugal separators, impingement or vane type separators and various systems employing combination of the above such as the separators utilizing both the impingement and cyclone principles.

While in the process described above the liquid feed is shown as being vaporized by means of indirect heat exchange with a hotter medium flowing countercurrent through tubeshell type heat exchangers, it is to be understood that fired heaters rather than exchangers employing counterflowing hotter mediums can equally be used. Indeed, the problem of fouling in a fired heater is more severe than it is in tube-shell or other counterflow type heat exchange systems and is equally likely to occur. In the preferred embodiment of the invention, however, wherein an alkyl aromatic feed is being vaporized prior to being introduced into a hydrodealkylation zone, the system as shown in FIG. 1 wherein the reactor effluent is used in successive stages of indirect heat exchange to heat the hydrodealkylation feed provides an extremely desira ble heat balance in the system.

What is claimed is:

1'. In a process wherein a liquid feed containing a major proportion of alkyl aromatic hydrocarbons is vaporized and the thus vaporized feed is further heated by indirect heat exchange, the improvement whereby said vaporized feed is rendered substantially free of entrained high boiling liquids comprising heating by indirect heat exchange said liquid feed to a temperature just below its dew point, introducing the heated feed into a separation zone wherein said high boiling liquids are separated by settling or impingement and removed from said heated feed and removing a substantially liquid-free gaseous feed from said separation zone.

2. The process of claim 1 wherein said liquid feed is heated to a temperature less than 10 F. below the dew point of said feed.

3. The process of claim 1 wherein said alkyl aromatic feed 5. The process of claim 4 wherein said hotter medium comand hydrogen are introduced concurrently into a hydrodealprises the reactor effluent from said hydrodealkylation zone. kylation zone after said alkyl aromatic feed is vaporized. 6. The process of claim 1 wherein said alkyl aromatic feed 4. The process of claim 3 wherein said liquid-free gaseous Comprises toluene and an alkyl naphthalene Concentratefeed from said separation zone is passed into indirect heat 5 The Process fclaim6wherein the dew P ofsaid alkyl exchange with a hotter medium to thereby raise the temperaaromatic feed is from about 450 to about 480 ture of said liquid-free gaseous feed. 

2. The process of claim 1 wherein said liquid feed is heated to a temperature less than 10* F. below the dew point of said feed.
 3. The process of claim 1 wherein said alkyl aromatic feed and hydrogen are introduced concurrently into a hydrodealkylation zone after said alkyl aromatic feed is vaporized.
 4. The process of claim 3 wherein said liquid-free gaseous feed from said separation zone is passed into indirect heat exchange with a hotter medium to thereby raise the temperature of said liquid-free gaseous feed.
 5. The process of claim 4 wherein said hotter medium comprises the reactor effluent from said hydrodealkylation zone.
 6. The process of claim 1 wherein said alkyl aromatic feed comprises toluene and an alkyl naphthalene concentrate.
 7. The process of claim 6 wherein the dew point of said alkyl aromatic feed is from about 450* to about 480* F. 